A patient may need ventilatory assistance as a result of disease and injuries of various kinds. The need can be direct, especially when the injury or illness afflicts the patient's respiratory system. The need can also be indirect, e.g. during anesthesia and some intensive care. The respiratory assistance can encompass everything from facilitating spontaneous breathing to total control of breathing. Mechanical ventilation (i.e., via a ventilator or respirator) is usually employed to provide the breathing assistance.
Mechanical ventilatory support is widely accepted as an effective form of therapy and means for treating patients with respiratory failure. Ventilation is the process of delivering oxygen to and removing carbon dioxide from the alveoli in the lungs. When receiving ventilatory support, the patient becomes part of a complex interactive system which is expected to provide adequate ventilation and promote gas exchange to aid in the stabilization and recovery of the patient. Clinical treatment of a ventilated patient often calls for monitoring a patient's breathing to detect an interruption or an irregularity in the breathing pattern, for triggering a ventilator to initiate assisted breathing, and for modifying or interrupting the assisted breathing periodically to wean the patient off of the assisted breathing regime, thereby restoring the patient's ability to breath independently.
Unfortunately, assistance provided by a ventilator does not precisely mimic normal ventilation. The normal mechanics of breathing are based on the active creation of a negative inspiratory pressure by the inspiratory muscles. Air is then sucked into the lungs during inhalation. Through this negative pressure in the lungs and thorax, an improved filling of the heart and increased cardiac output occur. Exhalation at rest is largely passive and follows after relaxation of the inspiratory muscles.
A problem that frequently occurs with long-term ventilator use is that the patient's inspiratory musculature becomes weakened. For example, mechanical ventilation (MV) has been shown to induce respiratory muscle dysfunction in animal models. Some studies have shown that controlled MV leads to significant losses (on the order of 25-50%) in diaphragmatic strength in as little as 12 hours in young, healthy animals. In fact, it has recently been shown that clinically significant diaphragm atrophy (˜50%) occurs in humans following as little as 18-69 hours of mechanical ventilation.
In many instances following MV, the patient then loses the ability to breathe spontaneously after the problems precipitating MV support have resolved. Risks associated with ventilator dependence include increased discomfort and risk of secondary diseases for the patient (such as pneumonia, pulmonary fibrosis, aspiration, acute renal failure, cardiac arrhythmias, sepsis, vocal fold dysfunction, and barotrauma), increased morbidity and mortality, high health care costs, and longer treatment duration times. Although patients with chronic ventilator dependency (CVD) comprise only 5% to 10% of patients in intensive care units, they consume approximately 50% of all ICU resources, as measured in staff time and equipment usage. Specifically, it has been estimated that weaning patients consumed about 41% of total ventilation time in intensive care unit patients. The economic cost of long term MV dependence is enormous. Many acute care facilities charge $2,000 to $4,000 per day for long term MV support. The annual national cost of patients on a ventilator for 2 to 3 weeks, excluding physician costs, has been estimated to be $1.3 to $1.5 billion. Episodes of long term MV dependency can financially devastate families and health care institutions and are a financial drain on private insurers and government health care resources. Unfortunately, acute care MV appears to be increasing at a rate of 50% per decade.
One known way of trying to simulate normal breathing mechanics is to stimulate the respiratory muscles (in particular the diaphragm), such as by the use of electrical stimulation or magnetic stimulation of the diaphragm. However, electrical stimulation carries a risk of injury to the muscle and can be painful whereas magnetic stimulation requires an expensive magnetic stimulating device and special training for the operator.
Other methods for treating MV dependence resulting from inspiratory muscle weakness include increasing spontaneous breathing trials, pressure support breathing, T-piece breathing, weaning protocols, the administration of growth hormones, and inspiratory resistance training. All of these methods produce limited strength gains and have not been successful in weaning more than about 50% of such patients from mechanical ventilation.
For example, with inspiratory resistance training, patients undergo training by removing ventilator support and attaching an IRT device to the patient's breathing tube with variable sized orifices, usually 0.5 to 5 mm. The theory behind this method is that the when the patient breathes through increasingly smaller orifices, the pressure required to sustain inspiratory airflow will increase, thus providing a progressively increasing strength training stimulus. In practice, however, this method has a fundamental flaw: the patients can consciously vary the pressure needed to sustain inspiratory volume by altering inspiratory airflow and thus the patients control the training stimulus rather than the providers. For example, if a patient is breathing through an IRT device with a 3 mm orifice and generates an inspiratory airflow of 45 liter/min, the pressure required to sustain that airflow will be much higher compared to when the patient breathes at the same 3 mm orifice setting with an inspiratory airflow of 30 liter/min. If the patient breathes through an IRT device with an inspired airflow of 15 liters/min, the pressure (and the strength training stimulus experienced by the inspiratory muscles) will be lower than when breathing at a flow rate of 30 liter/min. Patients are able to consciously sense the amount of muscular effort needed to sustain a volitional inspiration and interpret greater inspiratory pressure requirements as a more difficult muscular effort. Since patients are able to sense the amount of muscular effort needed to sustain inspiration, when they breathe through IRT devices, they will normally adopt very low inspiratory flow rates to minimize the conscious effort of breathing, i.e., make inspiring feel easier. While lowering the pressure required to generate inspiratory airflow by breathing with a lower flow rate will make the patient feel more comfortable, the reduced pressure requirement may be an inadequate stimulus for the inspiratory musculature to strengthen, thus lowering the effectiveness of IRT.
Despite the human and economic impact of prolonged MV dependence, there has been little research examining effective weaning treatment techniques in this population. Further, as noted above, there are no effective systems that are currently available to strengthen the inspiratory muscles and assist patients in weaning from mechanical ventilation.